Lithographic apparatus and device manufacturing method

ABSTRACT

Liquid is supplied to a space between the projection system of a lithographic apparatus and a substrate. A flow of gas towards a vacuum inlet prevents the humid gas from escaping to other parts of the lithographic apparatus. This may help to protect intricate parts of the lithographic apparatus from being damaged by the presence of humid gas.

This application is a continuation of co-pending U.S. patent applicationSer. No. 13/356,231, filed Jan. 23, 2012, which is a continuation ofU.S. patent application Ser. No. 13/323,404, filed Dec. 12, 2011, nowU.S. Pat. No. 8,810,771, which is a continuation of U.S. patentapplication Ser. No. 12/422,140, filed Apr. 10, 2009, now U.S. Pat. No.8,102,502, which is a continuation of U.S. patent application Ser. No.12/010,705, filed Jan. 29, 2008, now U.S. Pat. No. 7,532,304, which is acontinuation of U.S. patent application Ser. No. 10/961,395, filed Oct.12, 2004, now U.S. Pat. No. 7,352,433, which claims priority to Europeanpatent application EP 03256809.9, filed Oct. 28, 2003, each of theforegoing applications is incorporated herein in its entirety.

FIELD

The present invention relates to a lithographic apparatus and a devicemanufacturing method.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. Lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). Inthat circumstance, a patterning device, such as a mask, may be used togenerate a circuit pattern corresponding to an individual layer of theIC, and this pattern can be imaged onto a target portion (e.g.comprising part of, one or several dies) on a substrate (e.g. a siliconwafer) that has a layer of radiation-sensitive material (resist). Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively exposed. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion in one go, andso-called scanners, in which each target portion is irradiated byscanning the pattern through the projection beam in a given direction(the “scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.)

However, submersing the substrate or substrate and substrate table in abath of liquid (see, for example, U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate using a liquidconfinement system (the substrate generally has a larger surface areathan the final element of the projection system). One way which has beenproposed to arrange for this is disclosed in PCT patent application WO99/49504, hereby incorporated in its entirety by reference. Asillustrated in FIGS. 2 and 3, liquid is supplied by at least one inletIN onto the substrate, preferably along the direction of movement of thesubstrate relative to the final element, and is removed by at least oneoutlet OUT after having passed under the projection system. That is, asthe substrate is scanned beneath the element in a −X direction, liquidis supplied at the +X side of the element and taken up at the −X side.FIG. 2 shows the arrangement schematically in which liquid is suppliedvia inlet IN and is taken up on the other side of the element by outletOUT which is connected to a low pressure source. In the illustration ofFIG. 2 the liquid is supplied along the direction of movement of thesubstrate relative to the final element, though this does not need to bethe case. Various orientations and numbers of in- and out-letspositioned around the final element are possible, one example isillustrated in FIG. 3 in which four sets of an inlet with an outlet oneither side are provided in a regular pattern around the final element.

SUMMARY

The presence of liquid in a lithography apparatus results in thesurrounding gas (e.g., air) becoming very humid. Humidity levels of upto 100% are possible. Moisture in the gas can enter other parts of thelithography apparatus thus contaminating other machine parts andmeasurement components so the operation and accurate measurement of thelithography apparatus may become compromised. The moisture in the gasmay cause rusting of machine parts and therefore may reduce the lifespan of the lithography apparatus.

Accordingly, it would be advantageous, for example, to provide a methodof confining and/or removing humid gas.

According to an aspect of the invention, there is provided alithographic apparatus, comprising:

an illumination system configured to condition beam of radiation;

a support structure configured to hold a patterning device, thepatterning device configured to pattern the beam of radiation accordingto a desired pattern;

a substrate table configured to hold a substrate;

a projection system configured to project the patterned beam onto atarget portion of the substrate;

a liquid supply system configured to at least partly fill a spacebetween the projection system and the substrate with a liquid; and

a gas flow port configured to create a flow of gas to remove humid gasin a space above and in contact with the liquid, to confine the humidgas in the space, or both.

The humid gas above (where down is the direction of propagation of theprojection beam) the liquid may thus be confined to or removed from asmall volume relative to the projection apparatus. In an embodiment,there may be no rigid connection between parts of the apparatus sorelative movement between the parts of the apparatus may occur freely.The gas used should be clean and dry to avoid damage to the apparatusand to absorb the humidity.

In an embodiment, the gas flow port comprises a vacuum inlet, which mayalso remove a contaminant from the system. The contaminant removed canbe solid particles (which could damage the apparatus by scratching it),liquid particles or gaseous particles other than the gas itself. Thevacuum inlet may be annular shaped, the projection system being arrangedat the center of the annulus.

In an embodiment, the gas flow port comprises a passage through whichthe flow of gas flows. The passage bounds the volume of humid gas andthe clean, dry gas flowing through the passage helps to prevent thehumid gas from escaping. The passage may be formed at least partly by apart of the projection system and the gas flow port.

The lithographic apparatus may further comprise a cover, the coverforming a part of the passage, the cover being joined to the projectionsystem by a seal. The cover may thus provides a gastight cover to theprojection system, helping to prevent humid gas from entering theprojection system or the remainder of the lithographic apparatus. Theseal should be flexible and is, in an embodiment, a glue. Relativemovement between parts of the lithographic apparatus may therefore notbe compromised.

The liquid supply system may comprise a liquid confinement structureextending along at least part of the boundary of a space between theprojection system and the substrate. In an embodiment, the gas flow portis arranged so that a flow of gas is provided at least partly betweenthe liquid confinement structure and the projection system. Due to thepresence of the gas flow port, relative movement between the liquidconfinement structure and the projection system may take place. In anembodiment, the lithographic apparatus is arranged so that the gas flowport is arranged between the projection system and the liquidconfinement structure. The liquid confinement structure optionallycomprises a gas seal inlet configured to form a gas seal between theliquid confinement structure and a surface of the substrate. Humid gasis therefore confined by the substrate, the gas seal inlet, the liquidconfinement structure, the gas flow port and the projection system. Inan embodiment, the liquid confinement structure is mounted onto a baseframe of the lithographic projection apparatus. In an embodiment, theliquid confinement structure is movable relative to the base frame inthe Z, Rx and Ry directions (where the Z direction is the direction ofpropagation of the projection beam) but fixed in all other directions.

According to a further aspect of the invention, there is provided adevice manufacturing method, comprising:

providing a liquid to a space between a projection system and asubstrate;

flowing a gas in a space above and in contact the liquid to remove humidgas in the space, to confine the humid gas in the space, or both; and

projecting a patterned beam of radiation using the projection systemonto a target portion of the substrate through the liquid.

An embodiment of the invention easily may be used with the liquid supplysystem illustrated in FIGS. 2 and 3. Additional inlets and outletsarranged in a space above the liquid in those Figures would generate agas flow which could absorb the humid gas from or confine the humid gasin the gaseous space above the liquid.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,liquid-crystal displays (LCDs), thin-film magnetic heads, etc. Theskilled artisan will appreciate that, in the context of such alternativeapplications, any use of the terms “wafer” or “die” herein may beconsidered as synonymous with the more general terms “substrate” or“target portion”, respectively. The substrate referred to herein may beprocessed, before or after exposure, in for example a track (a tool thattypically applies a layer of resist to a substrate and develops theexposed resist) or a metrology or inspection tool. Where applicable, thedisclosure herein may be applied to such and other substrate processingtools. Further, the substrate may be processed more than once, forexample in order to create a multi-layer IC, so that the term substrateused herein may also refer to a substrate that already contains multipleprocessed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of 365, 248, 193, 157 or 126 nm).

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a projection beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the projection beam may not exactly correspond to thedesired pattern in the target portion of the substrate. Generally, thepattern imparted to the projection beam will correspond to a particularfunctional layer in a device being created in the target portion, suchas an integrated circuit.

A patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions; in this manner, thereflected beam is patterned. In each example of a patterning device, thesupport structure may be a frame or table, for example, which may befixed or movable as required and which may ensure that the patterningdevice is at a desired position, for example with respect to theprojection system. Any use of the terms “reticle” or “mask” herein maybe considered synonymous with the more general term “patterning device”.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, including refractiveoptical systems, reflective optical systems, and catadioptric opticalsystems, as appropriate for example for the exposure radiation beingused, or for other factors such as the use of an immersion fluid or theuse of a vacuum. Any use of the term “projection lens” herein may beconsidered as synonymous with the more general term “projection system”.

The illumination system may also encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents for directing, shaping, or controlling the projection beam ofradiation.

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts a liquid supply system according to an embodiment of theinvention;

FIG. 3 is an alternative view of the liquid supply system of FIG. 2; and

FIG. 4 is a detail of a lithographic projection apparatus according toan embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to aparticular embodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to provide aprojection beam PB of radiation (e.g. UV radiation).

a first support structure (e.g. a mask table) MT configured to hold apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device with respectto item PL;

a substrate table (e.g. a wafer table) WT configured to hold a substrate(e.g. a resist-coated wafer) W and connected to a second positioner PWconfigured to accurately position the substrate with respect to item PL;and

a projection system (e.g. a refractive projection lens) PL configured toimage a pattern imparted to the projection beam PB by patterning deviceMA onto a target portion C (e.g. comprising one or more dies) of thesubstrate W.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above).

The illuminator IL receives a beam of radiation from a radiation sourceSO. The source and the lithographic apparatus may be separate entities,for example when the source is an excimer laser. In such cases, thesource is not considered to form part of the lithographic apparatus andthe radiation beam is passed from the source SO to the illuminator ILwith the aid of a beam delivery system BD comprising for examplesuitable directing mirrors and/or a beam expander. In other cases thesource may be integral part of the apparatus, for example when thesource is a mercury lamp. The source SO and the illuminator IL, togetherwith the beam delivery system BD if required, may be referred to as aradiation system.

The illuminator IL may comprise adjusting means AM for adjusting theangular intensity distribution of the beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator ILgenerally comprises various other components, such as an integrator INand a condenser CO. The illuminator provides a conditioned beam ofradiation, referred to as the projection beam PB, having a desireduniformity and intensity distribution in its cross-section.

The projection beam PB is incident on the mask MA, which is held on themask table MT. Having traversed the mask MA, the projection beam PBpasses through the projection system PL, which focuses the beam onto atarget portion C of the substrate W. With the aid of the secondpositioner PW and position sensor IF (e.g. an interferometric device),the substrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the beam PB. Similarly, thefirst positioner PM and another position sensor (which is not explicitlydepicted in FIG. 1) can be used to accurately position the mask MA withrespect to the path of the beam PB, e.g. after mechanical retrieval froma mask library, or during a scan. In general, movement of the objecttables MT and WT will be realized with the aid of a long-stroke module(coarse positioning) and a short-stroke module (fine positioning), whichform part of the positioners PM and PW. However, in the case of astepper (as opposed to a scanner) the mask table MT may be connected toa short stroke actuator only, or may be fixed. Mask MA and substrate Wmay be aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2.

The depicted apparatus can be used in the following preferred modes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theprojection beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the projection beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT is determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the projection beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizes aprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Another liquid supply system solution according to an embodiment of theinvention is a liquid supply system with a seal member or a liquidconfinement structure which extends along at least a part of a boundaryof the space between the final element of the projection system and thesubstrate table. The seal member is substantially stationary relative tothe projection system in the XY plane though there may be some relativemovement in the Z direction (in the direction of the optical axis). Aseal is formed between the seal member and the surface of the substrate.In an embodiment, the seal is a contactless seal such as a gas seal.Such a system is disclosed in U.S. patent application Ser. No.10/705,783, hereby incorporated in its entirety by reference.

As shown in FIG. 4, a liquid reservoir 10 between the projection systemand the substrate is bounded by a gas seal 16 forming an annulus aroundthe projection system. The seal 16, in an embodiment a gas seal, isformed by gas e.g. air, synthetic air, N₂ or an inert gas, providedunder pressure via inlet 15 to the gap between seal member 12 and thesubstrate and extracted via first outlet 14. The over pressure on thegas inlet 15, vacuum level on the first outlet 14 and geometry of thegap are arranged so that there is a high velocity gas flow inwards thatconfines the liquid. The distance between the gas inlet and outlet andsubstrate W is small. The liquid reservoir is supplied with liquid byinlet 22 and extends above the bottom of the final element of theprojection system PL. Excess liquid is removed via outlet 14.

To prevent moisture-laden gas from pervading the entire apparatus, metalmembranes may be used to confine the humid gas between the projectionsystem and a liquid confinement structure used to at least partlycontain the liquid between the projection system and the substrate.Alternatively, rubber-like Viton fluoroelastomer rings may be used toconfine the gas. In such arrangements, however, it may be possible avibration is transmitted between the projection system and the liquidconfinement structure.

As shown in FIG. 4, the projection system also comprises a cover 35attached to a main part of the projection system PL by a seal 40. Theseal 40 should be flexible to accommodate small relative movementbetween the main part of the projection system PL and the cover 35.Glues are likely to be particularly effective. An advantage of such anarrangement is transmission of vibration forces between the projectionsystem and the liquid confinement structure may be reduced or avoided.

A vacuum chamber 34 with an inlet 33 is arranged in the volume above thereservoir 10. The outside of the vacuum chamber 34 and cover 35 form apassage 32 along which gas flows towards vacuum chamber 34. In additionto the gas flowing along passage 32, gas from all surrounding areas willflow towards the vacuum chamber 34. The partial vapor pressure of theliquid in the gas above the reservoir 10 is high, and the flow of gasalong passage 32 prevents the humid gas from entering the projectionsystem PL.

Additionally or alternatively, the gas flow will absorb humidity fromsurrounding gas so there is a gradient of humidity, the humidity of thegas decreasing away from the reservoir 10. Thus delicate parts of theapparatus such as mirrors for the interferometer beams are arranged in adry part of the apparatus so that the humidity doesn't affectmeasurements made using the interferometer beams. Additionally, if glueis used as seal 40, dry, flowing gas will help to ensure that the glueremains dry and therefore a gastight seal is maintained. Keeping theglue 40 dry also helps prevent it from expanding and thus generatingforces which may deform the projection system.

The vacuum chamber 34 can be independent of the projection system as inthe example above, or can be part of the projection system PL, or partof the seal member 12 and in any of these circumstances may beactuatable in the Z direction. There may be a plurality of gas passages32 and vacuum chambers 34 arranged around the projection system PL, oralternatively one annular (or other) shaped vacuum chamber with a slitinlet. Although the example here is of a vacuum chamber i.e. anunderpressure generating a gas flow, the gas flow port could equallycomprise an overpressure.

A system as described above may be used in conjunction with the liquidsupply system shown in FIGS. 2 and 3, the vacuum chamber 34 and passage32 being arranged above the inlets IN and outlets OUT.

In an embodiment, there is provided a lithographic apparatus,comprising: an illumination system configured to condition beam ofradiation; a support structure configured to hold a patterning device,the patterning device configured to pattern the beam of radiationaccording to a desired pattern; a substrate table configured to hold asubstrate; a projection system configured to project the patterned beamonto a target portion of the substrate; a liquid supply systemconfigured to at least partly fill a space between the projection systemand the substrate with a liquid; and a gas flow port configured tocreate a flow of gas to remove humid gas in a space above and in contactwith the liquid, to confine the humid gas in the space, or both.

In an embodiment, the gas flow port comprises a vacuum inlet. In anembodiment, the vacuum inlet removes gas in contact with the liquid. Inan embodiment, the vacuum inlet also removes a contaminant from gas incontact with the liquid. In an embodiment, the vacuum inlet has anannular shape, the projection system being arranged at the center of theannulus. In an embodiment, the gas flow port comprises a passage throughwhich the flow of gas flows. In an embodiment, the passage is formed atleast partly by the projection system and the gas flow port. In anembodiment, the apparatus further comprises a cover, the cover forming apart of the passage, the cover being joined to the projection system bya seal. In an embodiment, the seal is flexible. In an embodiment, theseal comprises glue. In an embodiment, the liquid supply system furthercomprises a liquid confinement structure extending along at least a partof the boundary of the space between the projection system and thesubstrate. In an embodiment, the gas flow port is arranged so that theflow of gas is at least partly between the liquid confinement structureand the projection system. In an embodiment, the liquid confinementstructure further comprises a gas seal inlet configured to form a gasseal between the liquid confinement structure and a surface of thesubstrate. In an embodiment, the liquid confinement structure is mountedonto a base frame. In an embodiment, the liquid confinement structure ismovable, relative to the base frame in Z, Rx and Ry directions and fixedin all other directions.

In an embodiment, there is provided a device manufacturing method,comprising: providing a liquid to a space between a projection systemand a substrate; flowing a gas in a space above and in contact theliquid to remove humid gas in the space, to confine the humid gas in thespace, or both; and projecting a patterned beam of radiation using theprojection system onto a target portion of the substrate through theliquid.

In an embodiment, the method comprises using a vacuum inlet to createthe flowing gas. In an embodiment, the method comprises using the vacuuminlet to remove gas in contact with the liquid. In an embodiment, thevacuum inlet has an annular shape, the projection system being arrangedat the center of the annulus. In an embodiment, the method comprisesflowing the gas through a passage, the passage formed at least partly bythe projection system and a device used to create the flowing gas. In anembodiment, a cover forms a part of the passage, the cover being joinedto the projection system by a seal. In an embodiment, the seal isflexible. In an embodiment, providing the liquid comprises providing theliquid to the space between the projection and the substrate using aliquid confinement structure extending along at least a part of theboundary of the space between the projection system and the substrate.In an embodiment, flowing the gas comprises flowing the gas between theliquid confinement structure and the projection system. In anembodiment, the method further comprises forming a gas seal between theliquid confinement structure and a surface of the substrate. In anembodiment, the liquid confinement structure is mounted onto a baseframe. In an embodiment, the liquid confinement structure is movablerelative to the base frame in Z, Rx and Ry directions and fixed in allother directions.

In an embodiment, there is provided a lithographic apparatus,comprising: an illumination system configured to condition beam ofradiation; a support structure configured to hold a patterning device,the patterning device configured to pattern the beam of radiationaccording to a desired pattern; a substrate table configured to hold asubstrate; a projection system configured to project the patterned beamonto a target portion of the substrate; a liquid supply systemconfigured to at least partly fill a space between the projection systemand the substrate with a liquid; a gas flow port configured to create aflow of gas to remove humid gas in a space above and in contact with theliquid, to confine the humid gas in the space, or both; and a passagethrough which the flow of gas flows formed at least partly by theprojection system and the gas flow port.

In an embodiment, the gas flow port comprises a vacuum inlet. In anembodiment, the vacuum inlet has an annular shape, the projection systembeing arranged at the center of the annulus. In an embodiment, theliquid supply system further comprises a liquid confinement structureextending along at least a part of the boundary of the space between theprojection system and the substrate. In an embodiment, the gas flow portis arranged so that the flow of gas is at least partly between theliquid confinement structure and the projection system.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

The invention claimed is:
 1. A lithographic apparatus, comprising: a movable table; a projection system configured to project the patterned beam onto a target portion of a substrate; a liquid supply system configured to at least partly fill a space between the projection system and the movable table with a liquid; a member that is located above the movable table and that extends along at least a part of the boundary of the space between the projection system and the movable table, the member configured to at least partly confine the liquid in the space; and a gas outflow port separate from the body of the member and configured to exhaust a flow of gas that at least partly confines humid gas in a volume around and in contact with the liquid in the space.
 2. The apparatus according to claim 1, wherein the gas outflow port comprises a low pressure inlet.
 3. The apparatus according to claim 2, wherein the low pressure inlet is configured to remove gas in contact with the liquid in a volume above the liquid.
 4. The apparatus according to claim 2, wherein the low pressure inlet has an annular shape, the projection system arranged at the interior of the annular shape.
 5. The apparatus according to claim 1, further comprising a cover to cover at least part of the projection system, the cover configured to prevent humid gas from entering the projection system or the remainder of the lithographic apparatus.
 6. The apparatus according to claim 5, further comprising a seal, wherein the seal is configured to seal the cover to the projection system.
 7. The apparatus according to claim 5, wherein the seal is flexible.
 8. The apparatus of claim 5, further comprising a passage between the member and the cover.
 9. The apparatus according to claim 1, wherein the gas outflow port is arranged so that the flow of gas is at least partly between the member and the projection system.
 10. The apparatus according to claim 1, wherein the member further comprises an outlet, on a bottom surface of the member, configured to remove liquid.
 11. The apparatus according to claim 10, wherein the member is movable in Z, Rx and Ry directions.
 12. The apparatus of claim 1, wherein the flow of gas is clean and dry gas.
 13. The apparatus of claim 1, further comprising a passage, between the member and the projection system, fluidly connected to the space.
 14. A device manufacturing method, comprising: providing a liquid to a space between a projection system of a lithographic apparatus and a movable table; confining the liquid in the space at least in part by a member of the lithographic apparatus located above the movable table; exhausting a flow of gas that at least partly confines humid gas in a volume around and in contact with the liquid in the space using a gas outflow port separate from the member; and projecting a patterned beam of radiation using the projection system onto a target portion of a substrate through the liquid.
 15. The method according to claim 14, comprising flowing the gas through a passage, the passage formed at least partly by the projection system and the member.
 16. A lithographic apparatus, comprising: a movable table; a projection system configured to project a patterned beam onto a target portion of a substrate; a liquid supply system configured to at least partly fill a space between the projection system and the movable table with a liquid; a member that is located above the movable table and that extends along at least a part of the boundary of the space between the projection system and the movable table, the member configured to at least partly confine the liquid in the space; a gas outflow port separate from the body of the member and configured to exhaust a flow of gas that at least partly confines humid gas in a volume around and in contact with the liquid in the space; and a gas inflow port configured to provide gas for the flow of gas.
 17. The apparatus of claim 16, wherein the gas inflow port is formed in a structure separate from the projection system and the member.
 18. The apparatus of claim 16, wherein the flow of gas is clean and dry gas.
 19. The apparatus of claim 16, wherein the member further comprises an outlet that is on a bottom surface of the member and that is configured to remove liquid.
 20. The apparatus of claim 16, further comprising a passage that is between the member and the projection system and that is fluidly connected to the space. 